1. Technical Field
This invention relates to a toner metering apparatus for an electrostatic reproduction apparatus and more particularly, to a toner metering apparatus for use in conjunction with a developer roll developer apparatus.
2. Background Art
In well known electrostatic printing processes, including electrophotographic or xerographic reproduction processes, an electrostatic latent image is formed on a moving charge retaining surface such as a photoconductor that repeatedly cycles through the reproduction process as the photoconductor is reused.
In the electrophotographic process, the first process step can be considered to be the full surface charging of the photoconductor to a uniform and usually quite high DC voltage, as the photoconductor moves past a charging station such as a charge corona. The charged photoconductor surface is then moved through an imaging station.
In a copier, the imaging station usually comprises an optical system that operates to reflect light off of an original document to be copied. As a result of the reflected light received from the document's white or lightly colored background area, the photoconductor retains a charge only in the area that corresponds to the document's darker or less reflective image area. This latent image is then toned, that is, covered with toner particles, as the photoconductor passes through a developing station. Since toner is applied to the charged latent image in a copier, the process is called a charged area development (CAD) process.
In a printer, the imaging station usually comprises a printhead that is driven by binary print data that is supplied by a computer. Laser printheads and LED printheads are two such well known imaging stations. Printers usually operate to discharge the photoconductor in the pattern of the image to be printed, that is, the printhead usually writes the image to be printed, and as a result the latent image comprises discharge areas of the photoconductor. However, printers can also be configured to write the background, in which case the latent image comprises a charged photoconductor area. In any event, this latent image is then toned, that is, covered with toner, as the photoconductor passes through a developing station. When toner is applied to the discharged latent image in a printer, the process is called a discharged area development (DAD) process. When toner is applied to the charged latent image in a printer, the process is again called a CAD process.
An additional electrostatic printing process employs a stationary array of charging elements which are selectively energized to form a charge pattern or image on a moving charge retaining surface. This surface is then toned as the surface passes through a developing station.
As will be apparent, the present invention relating to the development of a toned image on a charged surface finds utility in either a printer or a copier, and in either a CAD or DAD process. An embodiment of the invention to be described is that of a DAD printer.
The usual next step of either a copier or printer process is to transfer the toner image that is carried by the charged surface down-stream of the developer station to a transfer material such as paper. This is accomplished when the paper is supplied to a transfer station where it moves in actual contact or close proximity to the moving toned surface. As one side of the paper is in this close proximity of the toned surface, the other side of the paper is subjected to the action of a toner transfer station. Two well known transfer stations are roll transfer and corona transfer. In either event, an electrical charge is applied to the side of the paper removed from the toned surface so as to attract toner from the toned surface to the side of the paper in contact or proximity therewith.
Thereafter, the paper is separated from the moving surface and is transported to a fusing station whereat the toner is fused to the paper. The moving surface is then usually discharged and cleaned of residual toner in preparation for reuse in the reproduction or printing process.
Various development processes have been used for applying toner to the moving charged surface. One such technique employs a developer roller which is covered with toner in a manner to electrostatically charge the toner, rotated past a doctor blade to form a thin layer of charged toner on the developer roller and then rotated into contact or close proximity to the moving charged surface. The developer roller is electrically biased in a manner to form an electrical field so that the charged toner located on the surface of the developer roller adheres to the image pattern to be developed and is repelled from that area of the image pattern not to be developed.
Toner is supplied to prior art developer rollers by various processes. In one process, toner is added in bulk quantities to a sump in which the developer roller rotates from the sump area past the doctor blade to the photoconductor. The level of toner in the sump never exceeds a predetermined maximum level due to operator control of the level. When utilizing this developer system, toner particles having the smallest particle size tend to be utilized first leaving a toner mix of relatively large particle sizes. Thus, quality of the output copy deteriorates as toner is exhausted from the sump. Upon toner exhaustion, a new package of toner is added to the sump by the operator. A further problem with a sump system occurs when printing large black areas in one zone of the photoconductor. As toner is exhausted from the corresponding zone of the sump, light or uneven printing occurs in that zone.
A further aspect of the prior art involves the resupply of toner to a copier or printing machine to replace the toner exhausted in making copies. Typical prior art machines employed a mixture of toner and reusable carrier particles in the developing station. Various means were utilized to maintain a proper ratio of toner to carrier. Usually, the mixture was sampled and toner automatically added when required from a toner supply container. When toner was exhausted from the container, it was replaced with a new container of toner.
More recent prior art systems have utilized a cartridge which includes charging device(s), a developer system with a large supply of toner, the photoconductor, and a cleaning system for the photoconductor, all of which are discarded as a unit when the toner is exhausted. Such cartridge systems require a large supply of toner in order that the cartridge can be utilized to make a sufficient number of prints, hence making such cartridge systems economically feasible.
When such a large supply of toner is utilized with a developer roller system, large quantities of toner tend to accumulate in proximity to the doctor blade causing excess toner to be forced through to the photoconductor. This in turn causes more toner to adhere to image areas and possibly to non-image areas and consumes excess toner. Further, the pressure created at the doctor-blade developer roller interface due to large pile ups of toner causes the trapping of large toner particles at the interface and subsequent lack of toner in sections of the developer roller thus causing streaking on the print.